BACKGROUND OF THE INVENTION 1. Field of the Invention
The present invention relates to an electric motor, and particularly to a pneumatic bearing motor having pneumatic bearing structure.
2. Description of Related Art
In various precision instruments such as optical instruments, electronic instruments and the like, components need to have machining precision in the order of nanometer to meet a demand for high precision, high density and high integration. Machine tools, steppers and electron beam delineation devices for machining those components with high precision need to able to perform resolution with extremely high precision. In those machining and manufacturing devices, positioning is performed generally by a positioning device, and the position control by the positioning device is performed generally by a rotary servomotor or a linear motor which is controlled by a CNC. Therefore, in order to improve the machining precision of components, it is necessary to control the rotary servomotor or linear motor with high precision.
However, the rotary servomotor or linear motor usually has torque ripple, and therefore it is necessary to reduce the torque ripple in order to control the motor with high precision.
Torque ripple can be broadly classified into machine-structural torque ripple and electromagnetic torque ripple. For example, in the rotary servomotor, frictional resistance acting on bearings for a rotor shaft causes the machine-structural torque ripple, and magnetic distortion produced between a rotor and a stator causes the electromagnetic torque ripple.
In order to reduce the machine-structural torque ripple, it has been proposed to support a shaft in a non-contact manner with a pneumatic bearing or a magnetic bearing to thereby reduce the frictional resistance.
FIG. 17 is a schematic sectional view for explaining a conventional motor structure. As shown in FIG. 17, a stator 110 of a motor has portions of a winding which project from a stator core 111 in the axial direction of the motor, as denoted by reference numeral 104. Those portions are called xe2x80x9clugs of a windingxe2x80x9d, and do not contribute electromagnetic effect on a rotor. Because of those lugs, the size of the motor is made larger especially when a large number of turns of the winding are formed, to cause an obstacle to downsizing of the motor. In order to cope with this problem in downsizing the motor, there has been proposed a structure such that counterbores 132 are formed in a housing 131 for supporting the stator, by cutting off portions corresponding to the lugs 104 which are projecting portions of the winding, so that the lugs 104 may be received in the counterbores 132.
However, there is a problem that the presence of the counterbores 132 lowers the mechanical rigidity of the motor. Especially in the pneumatic bearing structure in which gas is supplied to a gap 140b between a rotor 120 and the stator 110 and gaps 140a between the rotor 120 and the housing 131 to thereby support the rotor 120 in a non-contact manner, the supplied gas exerts outward forces Fb and Fa on the stator 110 and the housing 131. A concentration of stress is produced by the forces Fb and Fa at portions of the housing 131 close to the counterbores 132 (denoted by A in FIG. 17) to cause distortion. The gaps may change in size by the distortion to make the support of the rotor 120 unstable, and when the distortion is made larger there is even a risk of breaking the housing 131.
Therefore, in the conventional pneumatic bearing motor, a structure having large thickness of the housing has been adopted so as to increase the mechanical rigidity. This structure makes it difficult, however, to downsize the pneumatic bearing motor.
An object of the present invention is to provide a pneumatic bearing motor capable of downsizing without lowering the mechanical rigidity thereof
According to one aspect of the present invention, the motor comprises: a rotor; a pneumatic bearing for rotatably supporting the rotor; and a stator having a slot-less stator core and a winding wound on the stator core layer by layer to form a toroidal shape so that a wire of the winding in each layer is not crossed.
In winding a wire helically to form the toroidal winding, the winding action is performed so that the wound wire does not form a crossing portion in each layer being formed by the winding action, thereby, positional precision of the winding is made high in a direction tangent to the surface of the stator core. Further, in a case of adapting the present invention to a linear motor, the winding is formed by winding a wire helically along an axis of the stator extending linearly.
In a regular winding, the winding is formed layer by layer and each layer is laid one on another, thereby positioning precision of the winding is made high in a direction of a normal line of the surface of the stator core. By improving the positional precision of the winding, the thickness of the whole winding is made uniform.
Thus, by winding a wire around a slot-less stator core to form a toroidal shape in a manner of the regular winding, the formed winding has high positional precision in both tangential direction and normal direction of the surface of the stator core, and the whole winding has a uniform thickness and does not have projections of lugs of winding as in the conventional winding.
The pneumatic bearing motor of the present invention adopting the above described structure of winding, since the stator does not have projections as exist in a conventional stator, it is not necessary to form counterbores in a housing or to make the wall thickness of the housing large for bearing stress concentration. Thus, it is possible to reduce the wall thickness of a housing for a pneumatic bearing motor and downsize the housing.
The winding is formed with high positional precision in the directions tangent and normal to the surface of the stator core, the winding shows uniform electrical resistance and uniform inductance, and therefore produces a uniform magnetic field.
According to another aspect of the present invention, the motor comprises: a rotor; a pneumatic bearing for rotatably supporting the rotor; and a stator having a slot-less stator core and a winding wound on the stator core to form a toroidal shape with a constant pitch. Thus, the winding formed by winding a wire into a toroidal shape at a constant pitch also produces the similar effects to those produced by the winding formed by winding a wire into a toroidal shape in the manner of the regular winding.
The stator may be provided with a covering molded of resin at its radial surface which faces the rotor, and the rotor may be provided with molded resin coating at its radial surface confronting the stator. With the coating, irregularities of surfaces of windings and magnets are compensated to make gas pressure uniform and secure stable support.
Further, with a mechanism for supplying gas to a gap between the stator and the radial surface of the rotor and gaps between the housing and the thrust surfaces of the rotor, large bearing surfaces are secured to thereby improve rigidity of the bearing to support the rotor stably.